1. Field of the Invention
This invention relates to a discharged-sheet conveying device in an image forming apparatus, such as a copier, a printer or the like, which uses an electrophotographic process, and more particularly, to a heat radiation control mechanism in a discharged-sheet conveying device which uses a thermal fixing process.
2. Description of the Related Art
FIG. 8 shows a conventional printer 100 which uses an electrophotographic process. In FIG. 8, there is shown a main body 1 of the printer. A cassette 2 for accommodating sheets of a recording material P is mounted on the right side of the main body 1. A single sheet of the recording material P is fed from the cassette 2 by a sheet feed roller 3. The sheet of the recording material P is fed between guides 5 and 6 to a photosensitive drum 9 at a predetermined timing by sheet feed roller 3 and a pair of registration rollers 7, and a toner image on the photosensitive drum 9 is transferred onto the sheet by a transfer roller 10. A process cartridge 11 includes a cleaning unit, a primary charger, a developing unit and the like. The sheet of the recording material P having the image transferred thereto is conveyed on a conveying guide 12 to a fixing unit 13.
The fixing unit 13 includes an entrance guide 13a and an exit guide 13b. A fixing roller 16 made of aluminum and including a halogen heater 15 is provided at an upper portion of the fixing unit 13. An elastic pressing roller 17 made of rubber is in pressure contact with the fixing roller 16 from below. Next to the exit guide 13b there are provided a pair of sheet discharge rollers 19, a flapper 20 for guiding the sheet of the recording material P in a horizontal or upward direction, and an upwardly directing sheet discharge guide 21 above the flapper 20. A sheet discharge roller 22 and a pair of uncurling rollers 23 are provided in pressure contact with each other at a horizontally directing distal end of the sheet discharge guide 21.
A sheet discharge tray 25 for facedown sheet discharge is provided on the upper surface of the main body 1 at a lower right side of the sheet discharge roller 22. A sheet discharge tray 26 for faceup sheet discharge which can be opened and closed outside the main body 1 and has a substantially horizontal posture when it is opened is provided at the left side of the sheet discharge rollers 19.
A scanner unit 27 scans laser light. A mirror 29 guides the laser light to the photosensitive drum 9.
A printing operation will now be explained. When a printing operation is initiated by a host computer (not shown) connected to the printer 100, the sheet feed roller 3 rotates to feed a sheet of the recording material P from within the cassette 2. The sheet is guided to the registration rollers 7 by the guide 6. The registration rollers 7 convey the sheet to the transfer position in synchronization with image information formed on the photosensitive drum 9 by the laser light. After an image has been transferred to the sheet, the sheet is fed to the fixing unit 13, where a fixing process is performed using heat and pressure. The flapper 20 is linked to sheet discharge tray 26 and moves in synchronization with opening/closing of the sheet discharge tray 26. When the sheet discharge tray 26 is opened (as indicated by two-dot chain lines), the flapper 20 is at a position indicated by broken lines. After the fixing process, the sheet is linearly conveyed, and is discharged and mounted onto the sheet discharge tray 26 with the image faced up (faceup sheet discharge) by the sheet discharge rollers 19. When the sheet discharge tray 26 is closed, the flapper 20 is at a position indicated by solid lines. The sheet is upwardly conveyed, and is discharged onto the sheet discharge tray 25 for facedown sheet discharge with the image faced down by the sheet discharge roller 22 and the uncurling rollers 23.
Since the sheet of the recording material P is subjected to a temperature between 150.degree. C. and 180.degree. C. and a pressure of about 7 kg in the fixing process, upward curling is produced along a nip shape formed by the fixing roller 16 and the pressing roller 17.
When the sheet of the recording material P is discharged onto the tray 26 for faceup sheet discharge, the sheet is conveyed with a slightly downward bent by the flapper 20 before the temperature of the sheet decreases. Hence, the sheet is discharged onto the sheet discharge tray 26 in a substantially uncurled state.
When the sheet of the recording material P is discharged onto the sheet discharge tray 25 for facedown sheet discharge, the sheet curl is increased by the outwardly convex sheet discharge guide 21 after the fixing process. After the curling of the sheet is reduced by an upwardly pressing force of the uncurling rollers 23 pressed against the sheet discharge roller 22, and the upwardly curved conveying path provided by the sheet discharge roller 22 and the uncurling rollers 23, the sheet is discharged and mounted onto the tray 25 for facedown sheet discharge.
However, since the recording material P may comprise paper or OHP sheets, the state of the generation of curling differs according to a difference in the property of the recording material used.
When paper is used, the amount of curling is increased as the temperature of the paper is higher if the paper is passed through a curved conveying surface.
A large degree of curling is produced particularly at temperatures higher than about 50.degree. C. It is possible to reduce or eliminate the curling by passing the paper through an inversely curved conveying surface. Alternately, the paper may be discharged after being conveyed in a flat state until the temperature of the paper is below 50.degree. C. (faceup sheet discharge).
When an OHP sheet is used, the shape of the sheet starts to deform at a temperature of between 55.degree. C. and 80.degree. C. The sheet is softened above 80.degree. C. and is hardened below 55.degree. C. Curling is not produced and hardly changes in any of the above-described conditions. Accordingly, in order to reduce curling in an OHP sheet, the sheet must be discharged in a high-temperature state, and must be cooled while stacking the sheet on a plane sheet discharge tray.
That is, when paper is used, either the paper must be subjected to faceup sheet discharge, or uncurling rollers must be added to a sheet discharge roller when the paper is subjected to facedown sheet discharge, since a sheet discharge guide promotes curling of the paper. Even when an OHP sheet is used, the sheet must be discharged before it cools down, that is the sheet must be subjected to faceup sheet discharge. Hence, conventional approaches have disadvantages in that the space for installing a printer will be increased due to the presence of a tray for faceup sheet discharge, and the production cost will be increased due to the provision of the tray for faceup sheet discharge or the uncurling rollers.